Science Focus 9
Electrical Principles and Technologies
Notes
Topic 6 - Generators and Motors
A device that converts mechanical energy (energy of motion – windmills, turbines, nuclear power,
falling water, or tides) into electrical energy is called an electric generator. The operation of a
generator depends on the relationship between electricity and magnetism.
Electricity to Magnetism
Deflection of a compass needle using electrical current showed that there is a relationship
between electricity and magnetism. Hans Christian Oersted found that the current created a
magnetic field around the wire. The amount of needle deflection depended on how much electric
current was flowing in the wire. When the current was reversed, the needle moved in the
opposite direction.
Electromagnets
When a soft iron core is inserted into a coil of wire and a current is
passed through the wire, a very strong temporary magnet is produced,
called an electromagnet.
The strength of an electromagnet is affected by the …
ƒ type and size of core
ƒ strength of current
ƒ number of coils
Magnetism to Electricity
Electric effects can also be created using a magnet. Michael Faraday
(and Joseph Henry) discovered electromagnetic induction in 1831. They
demonstrated that moving a conducting wire through a magnetic field by
moving it back and forth through the field, Faraday created the first
electricity-producing generator, which could generate electrical current.
They also found that moving the magnet worked as well.
Faraday introduced terms such as ‘ion’, ‘electrode’, ‘cathode’, and ‘anode’
to science and invented the lines of magnetic force. The farad, a unit for
measuring stored electric charge, was named after him.
What’s in a Generator?
An AC generator – the most common type – has a coil of wire rotating inside a stationary field
magnet. The central axle of an AC generator has a loop of wire attached to two slip rings. The
current is switched as the loops move up and down alternatively through the magnetic field. The
slip rings conduct the alternating current to the circuit through the brushes (the brush and ring
assembly allows the whole loop to spin freely).
The electricity produced by this type of generator is called alternating current because it changes
direction (in North America it changes direction 120 times per second – giving 60 Hertz or
complete waves each second. In large AC generators many loops of wire are wrapped around an
large iron core. Massive coils of wire rotating in huge generators can produce enough electricity
to power an entire city.
Science Focus 9
Electrical Principles and Technologies
Notes
DC Generators
A DC generator is much the same as a DC motor, and is often called a dynamo. The spinning
armature produces the electricity (if electricity is passed through a DC generator, it will spin like a
motor). The armature is connected to a split ring commutator which enables this type of
generator to send current through a circuit in only one direction. The DC generator’s pulsating
electricity is produced in one direction - referred to as direct current - and coincides with the
spinning of the generator.
The St. Louis motor was designed to teach how a DC motor works
Electric Motors: Electric to Mechanical Energy
An electric motor is constructed in exactly the same way as a
generator. Instead of producing electricity, it uses electrical energy to
make a wire coil spins between the poles of a magnet. Current flowing
through the coils makes it an electromagnet, which is then affected by
the laws of magnetic forces when it is in proximity to the field magnet.
Opposite poles attract and like poles repel. All electric motors operate
on this principle.
Some motors run on direct current (DC). It is 'direct', because the electricity flows in only one
direction. Alternating current (AC) flows back and forth 60 times per second
DC Motors
Faraday constructed the first motor. By coiling (copper) wire around a (iron) metal core a strong
electromagnet can be made. When attached to an electrical source it will produce a strong
magnetic field. To keep this electromagnet spinning in a magnetic field, the direction that the
current is traveling through the coil must be switched. This is accomplished by with a gap, which
allows the polarity of the electromagnet change just before it aligns with the permanent magnet.
DC motors use a commutator (a split ring that breaks the flow of electricity for a moment and then
reverses the flow in the coil, when the contact is broken, so is the magnetic field) and brushes
(contact points with the commutator) to reverse the flow of electricity through the magnetic field.
The armature (the rotating shaft with the coil wrapped around it) continues to spin because of
momentum, allowing the brushes to come into contact once again with the commutator.
AC Motors
AC motors have a rotating core, or rotor, made up of a ring of non-magnetic conducting wires
connected at the ends and held in a laminated steel cylinder. Surrounding the rotor is a
stationary component called a stator. The stator is a two-pole electromagnet. When the motor is
turned on, the attraction and repulsion between the poles of the stator and the rotor cause the
rotor to spin.